In monopulse antennas, a plurality of radiation patterns are made use of simultaneously and the shapes of the patterns have a direct bearing on the overall performance of the radar system employing the antennas. In monopulse techniques, simultaneous use is in fact made of a plurality of patterns originating from the same antenna. In so-called amplitude operation, for example, a distinction is made between on the one hand a pattern of even symmetry or sum diagram, which acts as a reference, and on the other hand patterns of odd symmetry or "difference" patterns which provide signals which represent deviations in azimuth and elevation from the axis of the antenna.
In so-called "phase" operation, the signals for the angular deviations are obtained by a phase comparison between two patterns having the same amplitude characteristic. It should incidentally be mentioned that it is possible to change over from one mode of operation to the other by means of a coupler system and it will therefore only be the case of amplitude operation which will be considered in the following description.
In these various modes of operation, the patterns employed can be represented mathematically by orthogonal functions, which means that the corresponding channels are decoupled. However, the various radiation characteristics of these patterns, which have a direct bearing on the performance of the system, are not independent of one another a priori but are related by limiting relationships depending on the structure of the antenna. These characteristics are gain and the level of the side lobes on the sum channel and the difference channels, the slope close to the axis, and the gain of the main lobes on the difference channels.
For a given antenna structure, the problem which is posed comes down to finding an optimum balance between the factors which have been mentioned while keeping them in the order of importance imposed by the functions of the system concerned. It can be deduced from this that any structure has a range where it is optimum but it is precisely in the case of monopulse techniques that conventional antenna structures have revealed their limitations. In conventional monopulse antennas, it has in fact been found impossible to control the sum pattern and the difference patterns independently of one another, or to control properly the form of the illumination characteristic of the primary feed which is of importance mainly in the construction of low-noise antennas for radio-astronomy and space telecommunications. The limitations of conventional monopulse techniques have also been shown up when they have been applied to antennas for tropospheric-scatter communication where diversity between the "sum" and "difference" channels is utilized.
To overcome these limitations, so called multimode feeds have been developed and are used in antennas which are also termed multimode.
By virtue of the structure with which it is endowed, a multimode feed, also called a moder, is capable of generating direct propagative modes whose phases and amplitudes can be controlled to allow a desired illumination to be obtained in its aperture.
In general terms, a moder is a structure formed from waveguides containing discontinuities designed to generate upper modes.
A study of such moders may be found, inter alia, in French Pat. No. 1,290,275, of whose FIG. 1 will be reviewed herein, and which relates to a mixed-multimode structure representative of the prior art which is formed by combining an E-plane moder and an H-plane moder in the way shown in FIG. 1.
Such a structure enables independent control to be achieved of the sum and difference patterns in the E plane and the H plane. However, such control is not exercised simultaneously in the E and H planes but successively in the E plane and then the H plane.
The structure shown in FIG. 1 is formed by two plane moders ME.sub.1 and ME.sub.2 which are positioned side by side and which are separated by a common vertical partition. The moders are each excited by one of two pairs of guides 1, 10 and 2, 20 which receive the fundamental mode and which each open into a guide 3, 30 having a length L.sub.1 between planes P.sub.0 and P.sub.1. Plane P.sub.0 is called a plane of discontinuity at which upper, propagative or evanescent, modes are formed, the length L.sub.1 and the dimensions of guides 3, 30 being such that only the desired modes, which in the present instance for example are the odd H.sub.11 and E.sub.11 modes and the even H.sub.12 and E.sub.12 modes, are propagated to the aperture of the E moder so formed, that is to say plane P.sub.1, the fundamental mode of excitation being the H.sub.10 mode.
Following on from plane P.sub.1 are H-plane moders which will produce the desired patterns of distribution in the horizontal plane without upsetting the distribution patterns produced in the vertical plane by the E moders ME.sub.1 and ME.sub.2.
Metal plates 4, 40, 5, 50, 6, 60 are arranged horizontally in a guide 8, 80 of length L.sub.2 which forms a continuation of guides 3 and 30 beyond plane P.sub.1 and these plates define four pairs of flat horizontal guides which are adjacent at their small sides and which are excited with the patterns of distribution defined by moders ME.sub.1 and ME.sub.2. The horizontal plates extend past plane P.sub.2 into a guide 7 of length L.sub.3 in the shape of a horn.
The assembly situated between planes P.sub.1 and P.sub.3 forms a set of superimposed H-plane moders, plane P.sub.2 being the plane of discontinuity at which the upper modes are formed. The aperture of the mixed structure, which is situated in plane P.sub.3, radiates with an overall illumination characteristic which is the product of the partial illumination characteristics obtained in the vertical plane and the horizontal plane.
Multimode sources conforming to what has just been described are used in antennas but they have the drawback of being of considerable size longitudinally, which is a hindrance when producing certain antennas where any increase in performance, in particular in respect of pass band, results in an increase in inertia which has an adverse effect on the operation of the servo-mechanisms.